Sources told ABC News a backpack in Melaku's possession contained a package labeled ammonium nitrate as well as spent firearm ammunition. Ammonium nitrate, a common fertilizer, can be used in explosives. The FBI later said despite the label, the backpack presented no immediate threat. A search of Melaku's nearby car also found no potential devices. Law enforcement sources said Melaku does not appear to have known ties to any terror organization. In addition to his arrest today, Melaku already faces pending charges in Loudoun County, Virginia, for a rash of vehicle break-ins, police said.